Since the Multiple-Input Multiple-Output (MIMO) technologies are important to an improvement of a peak rate, and a utilization ratio of system spectrums, all the radio access technology standards of Long Term Evolution (LTE), LTE-Advanced (LTE-A), etc., are built based upon the MIMO plus Orthogonal Frequency Division Multiplexing (OFDM) technologies. A performance gain of the MIMO technologies arises from a space freedom available in a multi-antenna system, so one of the most important evolvements of the MIMO technologies being standardized is an extension of dimensions. In the LTE Rel-8, MIMO transmission of at most four layers can be supported. In the Rel-9, primarily the Multi-User MIMO (MU-MIMO) technologies have been enhanced, and at most four downlink transmission data layers can be supported in MU-MIMO transmission of the Transmission Mode (TM)-8. In the Rel-10, an 8-port Channel State Information-Reference Signal (CSI-RS), a User Equipment (UE)-specific Reference Signal (URS), and a multi-granularity codebook have been introduced to improve a space resolution of channel state information, and to further extend a transmission capacity of Single-User MIMO (SU-MIMO) to at most eight transmission data layers.
In a base station antenna system structured as a traditional Passive Antenna System (PAS), a plurality of antenna ports (each port corresponds to a separate radio frequency-intermediate frequency-baseband channel) are arranged horizontally, and a plurality of array elements in the vertical dimension corresponding to each port are connected through a radio frequency cable, so with the existing MIMO technologies, only space characteristics of signals of respective UEs in the horizontal dimension can be optimized by adjusting relative amplitudes/phases between the different ports in the horizontal dimension, and only uniform sector level beam-forming can be performed in the vertical dimension. After the Active Antenna System (AAS) technologies have been introduced to a mobile communication system, the base station antenna system can be provided with a higher freedom in the vertical dimension, and can optimize a signal at a UE level in the three-dimension space.
Further to the research, the standardization, and the development of the antenna technologies, the MIMO technologies are being further advanced in the industry toward becoming three-dimensional and large-scale. At present, there is a research project ongoing in the 3GPP on 3D channel modeling, and there are also researches and standardization efforts to be conducted as expected on the technologies of Elevation Beam Forming (EBF) of eight or less antenna ports, and Full Dimension-MIMO (FD-MIMO) of more than eight ports (e.g., 16, 32, or 64). Researches and test efforts have been conducted from the foresighted perspective in the academic community on the massive MIMO technologies based upon a larger-scale antenna array (including one or more hundreds and even more array elements). The academic researches, and the results of the preliminary channel in-situ measurements have showed that the massive MIMO technologies can greatly improve the utilization ratio of system bands, and support a larger number of accessing UEs, so the massive MIMO technologies are expected by the major research organizations to be one of the most promising physical layer technologies in a next-generation mobile communication system.
In the massive MIMO system, as there are an increasing number of antennas, the quality of transmitting data over a service channel, and the capability to suppress interference to the data have significantly benefited from the high space resolution of pre-coding/beam-forming arising from the extended array scale. However as a UE is moving at a very high speed, Channel State Information (CSI) reported by the UE may not be sufficient to follow up a varying condition of a channel, so it may be difficult at the base station side to match with the channel of the UE precisely in a timely manner, thus resulting in a serious loss of performance while data are being transmitted.
In summary, as a UE is moving at a very high speed, CSI reported by the UE may not be sufficient to follow up a varying condition of a channel, and if data are transmitted based upon the CSI reported by the UE, then there may be a serious loss of performance.